Woad (istatis tinctoria) is a hardy biennial plant native to Britain and parts of Europe. Julius Caesar in Book Five of his Gallic Wars mentions that the British tribes used to daub themselves in a blue-indigo dye. However, it was used mainly as a dye in the textile industry.

The flowers and leaves were picked between June and September and were left to ferment before being processed, usually in a mill specially designed for extracting the dye. It was still cultivated for industrial purposes in England until the 1930s.